Simple threaded fasteners include a male threaded component and a female threaded component configured to engage one with another to affix multiple items to each other. According to the Industrial Fasteners Institute (IFI), a thread is defined as a uniform section in the form of a helix on the external or internal surface of a cylinder. It is known to provide either straight or parallel threads formed on a cylinder or tapered thread formed on a frustum or cone. Threaded fasteners are used in widely differing applications, environments and conditions for widely varying purposes. Threaded fasteners are known to be made of many different materials, both soft and hard, including various metals and plastics.
To improve the performance of threaded fasteners for different purposes, a variety of thread configurations are known such as machine thread forms and spaced thread forms. In general, threads of all types are based on a straight-line helix pattern. The thread pitch may vary from one type or size of fastener to another, and it is known to provide a differing thread pitch on different portions of the same fastener. However, regardless of the pitch the thread follows a straight-line helical pattern.
In a standard fastener design, for a given or “basic” pitch, the male thread is provided at equal to or less than the basic pitch, and the female thread is provided at equal to or more than the basic pitch. The result is that the male thread “floats” within the female thread, allowing the two components to be run together throughout the thread length with little or no interference until clamping pressure is applied during final tightening of the fastener.
As the fastener is tightened and clamp load applied, friction is created from stretch in the fastener as it is placed under tension. Due to vibration, slip of the angular thread surfaces of the fastener over time, expansion and contraction cycles and the like, the clamp load can loosen. It is known to provide prevailing torque in the fastener by deforming the nut or using paste-like substances in the threads to maintain the relative position of the male and female components, even if clamp load is lost. Crimping a nut adds considerable cost to the manufacturing process, and known paste-like substances for the purpose are somewhat limited in extreme temperature conditions and have limited reusability. Prevailing torque thread forms, including plastic inserts typically in the female thread, rely on the crest or root of one fastener interfering with the mating thread of the other fastener. Reliability and repeatability of performance have not been achieved with these designs. Further, prevailing torque provided in this fashion is present throughout the length of threaded engagement between the male and female component, requiring additional torque throughout rotation. Prevailing torque constructions have been undesirably expensive to manufacture.
It is generally accepted in fastener design that up to 80% of the all tension is absorbed in the first three threads of the fastener. Accordingly, fastener designers must account for this feature. It would be advantageous to distribute tension more evenly throughout the entire length of the engaged fasteners, or at least more substantial portions thereof.
Further difficulties occur when threaded fasteners are used in somewhat non-standard situations. Tightening the fastener requires application of clamp load, meaning the head of the fastener, the confronting surface of the female fastener and all materials or components therebetween are compressed together. In so called “soft joints” such as, for example, joints holding together elastic materials, gaskets or the like, it is desirable that the fastener joint be tight without excessive clamp force applied on the material being held. Shoulder bolts or standard fasteners with spacers have been used for this purpose, complicating assembly and increasing costs.
Threaded fasteners are known to be preassembled in components that are designed to be installed with other components or associated members, and thereafter tightened. For example, various electrical assemblies are provided for use in the field with screws already in place on terminals to receive wires therein. With the wire properly positioned, the screw is tightened to establish electrical connection between the wire and the electrical assembly. The pre-installed position of the screw must be relatively secure so that the screw does not become lost, making the component unusable. Various interference or prevailing torque configurations are known to hold the screw position, but have the undesirable effect of increasing the torque required for turning the screw throughout its length. This makes preassembly and final tightening more difficult. It is also know to preassemble such devices by running the screw in until it bottoms out and can be tightened. This has the disadvantage of requiring that the screw is backed out before installation and use can commence. This can be both time consuming and difficult.
What is needed is a screw thread form that provides prevailing torque to keep fasteners tight and that can be used to preposition a male fastener relative to a female fastener for preassembled field ready components.